1. Field of the Invention
This invention relates to a method for preparing 2,6-bis (picrylamino)-3,5-dinitropyridine (PYX) which is a reliable explosive material of improved effectiveness as compared to the PYX prepared by methods previously in use.
2. Brief Description of the Prior Art
It has characteristically been difficult to initiate the detonation of many high temperature explosives. This problem of detonation initiation difficulty may be quantified by the results of a test which is referred to in the art as "air gap" sensitivity. A well known, widely used high temperature, thermally stable explosive and propellant is 2,6-bis(picrylamino)-3,5-dinitropyridine (hereinafter referred to as PYX). PYX exhibits relatively poor detonation initiation under some conditions of use.
This commercially available material is used to manufacture high temperature oil and gas well perforators and detonating cord. The perforators and detonating cord are typically fixed inside tubular perforating guns lowered into the well. Several guns may be connected together axially. Where such is the case, the gun-to-gun interconnection schemes utilized at present require that a detonating cord extend from one gun to another, and be able to propagate its detonation across air gaps in the tandem assembly of guns, thereby making its firing connection to the next gun in the series. Failure of detonation transfer frequently occurs at these air gaps, but such failures can be reduced to some degree by placing boosters of purified PYX at the air gap interfaces. Another frequent site of failure of detonation continuity occurs where the detonation must be propagated from the detonating cord to the perforator, and even here the slightest misalignment between the cord and the perforator may result in failure. It has been experienced that the purified PYX which is commercially available is not adequately sensitive to prevent and eliminate misfires occurring at the described sites. There is, accordingly, a need to improve the PYX so as to increase its shock sensitivity, and to reduce the frequency of detonation failure when this otherwise excellent explosive material is utilized.
It is also known that both the shape and size distribution of crystalline explosive particles are often important characteristics in determining the safety with which the explosive can be used, its sensitiveness, and the uses to which it can be most effectively put. Because of such considerations, explosives are produced by various procedures in an endeavor to achieve the explosive product in an optimum physical form in terms of the sought characteristics and the intended use. Variations in the formulation technique result in variations in the granulation and crystal morphology of the explosive product crystals, and result in improvement in the powder flow, bulk and pressed density, and in the dusting properties of the explosive.
In the case of PYX, the basic process by which this explosive was originally produced commercially is described in Coburn U.S. Pat. No. 3,678,061. Certain disadvantages and difficulties attended the Coburn method of PYX production, however, including the production of undesirable corrosive by-product acids as the two-step reaction proceeded, and the production of a final PYX product which demonstrated less than optimum thermal stability.
A subsequent effort to improve the properties of the PYX product is described in European Patent Application No. 0 104 717 of Hudson, published Apr. 4, 1984. The Hudson European Patent Application, as published, described a procedure involving two reaction steps, in the first of which 2,6-diaminopyridine is reacted in a polar protic solvent with a picrylating compound in the presence of an alkaline earth metal carbonate to yield as an intermediate compound, 2,6-bis(picrylamino)pyridine. By this procedure of producing the intermediate compound, the production of undesirable by-product acids is substantially reduced as compared to the yield of such acids in the practice of the Coburn process, and the presence of an alkali or alkaline earth carbonate or bicarbonate effectively removes the small amount of such undesirable product as is produced.
The intermediate compound, 2,6-bis(picrylamino)pyridine is readily isolated in good yields, and extensive purification is not required prior to performing the second reactive step which entails the nitration of the intermediate compound with concentrated nitric acid. The rapid separation of the intermediate which can be realized enables the nitration step to be carried out without the need for recrystallization of the intermediate prior thereto.
Following nitration of the intermediate with concentrated nitric acid, a yield of the order of ninety percent is realized, and the PYX crystals produced have a higher thermal stability than those which are produced according to the method of Coburn U.S. Pat. No. 3,678,061.
This procedure for producing PYX in which the initial reaction is carried out in a polar, protic solvent in the presence of an alkali metal bicarbonate is also described in J. C. Adenis et al "Synthesis and Properties of Some Thermally Stable Initiating Explosives" European Space Agency, ESA SP-144, 1980, pages 69-76.
A yet more recent technique used for modifying the form of crystalline PYX explosive is that of recrystallization or reformation from certain specific types of solvent systems. In Pallanck U.S. Pat. No. 4,564,405, a method is disclosed for producing improved, thermally stable PYX by treating certain crystalline forms of PYX with dimethylsulfoxide (DMSO) to form an intermediate adduct compound. The adduct compound is then dissociated into either cubic crystalline particles or an agglomerate of spherical particles of PYX material. The addition compound of DMSO is formed by adding one molar proportion of PYX to about thirty moles of solvent DMSO at a temperature of from about 100.degree. C. to about 110.degree. C.
The resultant solution is cooled to obtain a slurry of the adduct crystals, and these are then recovered by filtration, and are then dissociated by one of several methods. The method of dissociation used will determine what type of PYX product is formed. In one type of dissociation, a solvent which is miscible with DMSO, but in which the PYX has only very slight solubility, is added to the DMSO to cause the precipitation of an agglomerate form of PYX. Alternatively, cubic crystalline particles of PYX are obtained by subjecting the recovered adduct crystals to heat or vacuum, or a combination of heat and vacuum.
The PYX material derived from the DMSO adduct is a superior explosive as compared to other commercially available needleshaped crystals of PYX because it has improved thermal stability, and is of increased density. The uniformity of the particles also makes the product easy to pour and to handle, and it has a reduced tendency to acquire a static charge. The process described in Pallanck U.S. Pat. No. 4,564,405 has enhanced the sensitivity of the PYX produced thereby, and thus has enabled detonation failures at air gap interfaces to be reduced. Such failures are not completely eliminated, however, when PYX produced by the Pallanck process is used, and moreover that material is not available in large quantities.